1. Field of Invention
The present invention relates to broadband wireless access systems, and amongst other things to a method of dynamically scheduling multiple upstream channel transmission in a broadband wireless access system.
2. Discussion of Background
Point to multi-point fixed broadband wireless access systems over MMDS networks are known in broadcast situations. These networks operate over licensed bands including the MMDS band (2,150 to 2,162 MHz), the WCS band (2,305 to 2,360 MHz) and the ITFS/MMDS bands (2,500 to 2,686 MHz).
A known cable based broadband access system, which operates at a range of between 50 MHz and 864 MHz, but not in the MMDS, WCS, or ITFS/MMDS bands, is the data over cable specification system, which is specified in the data over cable system interface specifications (DOCSIS). An overview of the cable based DOCSIS system is depicted in FIG. 1. A CMTS 10 communicates with a wide area network 20, such as the internet. The CMTS 10 can transmit signals from the wide area network 20 along a cable network 30 through cable modems 40 to CPE 50 (Customer Premise Equipment—intended throughout this document to include a computer and/or all of the equipment at the customer site, such as a LAN—Local Area Network). CPE 50 messages can be transmitted to the wide area network 20 through the cable modem 40 along the cable network 30 to the CMTS 10.
In point to multi-point broadband access systems one central end-point, i.e. the head-end, communicates through a bi-directional link or links with multiple end-points, i.e. the nodes. The number of nodes in communication varies in time and can be none, one or two or more at any specific time.
The link(s) between the head-end and the nodes are combined in one or more channels. The signal path from the central end-point to the nodes is referred to as downstream, while the signal path from the nodes to the central end-point is referred to as upstream.
A single channel can be used to deliver information from a node to the head-end, and the downstream is used from the head-end to a node or a group of nodes. On any single upstream channel used for communication from the nodes(s) to the central point, then only one end-point can successfully send information on the single upstream channel at any one time.
If multiple upstream channels are utilized for communication from the nodes(s) to the central point the upstream channels need to be differentiated from one another to prevent interference. Examples of differentiation methods include FDMA, different sub-channels of OFDM, transmission of different polarity or direction of the node antennas, and different subnets in a CATV network.
To successfully utilize several upstream channels, it is required that no two nodes transmit on the same upstream channel during the same time period, e.g. time slot in a TDMA type scheme. Therefore, a coordination method is required that will allocate to each node a separate channel and/or time interval to prevent interference.
The performance characteristics, including such factors as capacity and error rate, can be different for each upstream channel and for each node that is using the upstream channel. These performance characteristics generally vary over time, due to environmental conditions. Examples of specific causes of the variations are multi-path fading or external interference, which are both frequency dependent. Different channels can use different parameters, such as modulation scheme and symbol rate, to support more robustness or higher information rate.
A known upstream channel allocation scheme is defined in DOCSIS. These specifications refer to the case of HFC network. In the DOCSIS system, each upstream channel is assigned a different frequency range. Different channels are used for upstream or downstream directions.
In the DOCSIS scheme there is no need to coordinate the downstream channel, since only the head-end is transmitting in this direction. Further in DOCSIS, the head-end is responsible for the allocation of the upstream channels. These allocations are performed in two general steps, one for the allocation of an upstream channel and the other for the allocation of time intervals in the upstream channels.
The head-end periodically transmits information regarding available upstream channels and their parameters on the downstream channel. This information is used during the registration process by the modem to select an upstream channel. Also, the head-end can command a node to change its upstream channel. Changing of an upstream channel is a long procedure, which may take a long period of time, while the service is interrupted. Hence, a node normally maintains the same upstream channel for a long period of time or even the length of its entire communication scheme.
The allocation of the time intervals on each upstream channel is also performed by the head-end. The head-end transmits the time interval allocations on the downstream channel in a message called MAP. A single MAP message describes time interval allocation on a single upstream channel for a specific period of time.
The DOCSIS solution uses a fixed upstream channel for each node, which implies that statistical changes to the traffic load may cause a high load on one channel, while not allowing other channels with lower loads to be used to balance higher load channel. Further, if the performance of the current upstream channel of a node becomes unacceptable, e.g. falls below predetermined threshold levels, the node must switch to an alternate channel. This switching process, which includes a search for the best available channel, takes a longtime during which service to the node is interrupted.
It is desired to make the upstream channel allocations in such a way that will utilize as much of the upstream channel capacity as possible. Further, it is desired that should communication with each node on an upstream channel is maintained as having as high of performance capabilities as possible. Additionally, when multiple services are supported by the broadband access system, some Quality of Service (QoS) requirements need to be defined which add additional limitations on any allocation scheme. Such limitations can include the committed information rate for each node.